Preliminary MSL Entry, Descent & Landing Reconstruction

August 10, 2012

The Mars Science Laboratory Entry, Descent & Landing Team has presented its initial results of its EDL analysis, so that a fair reconstruction of events is now possible. For those interested in the pure results, skip the article and go right to the timeline!The times and data that were presented during Friday's press conference have all originated in the 8kbps UHF Telemetry that was transmitted by Curiosity on the way through the Martian atmosphere all the way to touchdown and beyond. During EDL, the Deep Space Network Station in Canberra and ESA's New Norcia Station picked up 67 Heartbeat Tones of the Spacecraft and 26 Status Tones before Earth occultation occurred on Mars at approximately EI+5 minutes just after Heatshield Separation. The Mars Reconnaissance Orbiter was able to record UHF telemetry from 8 minutes prior to Entry Interface to touchdown +6 minutes while Mars Odyssey provided bent-pipe relay from EI+2:19 for about 12 minutes.

Now, we look at the major milestones and assess how well our EDL Timeline fared.It all starts with Entry Interface which is essentially THE Reference Point of all EDL Events as it marks the start of the actual Entry into the Martian Atmosphere. The time of EI was available with full accuracy after the last Trajectory Correction of MSL was performed on July 29. This reference was taken to put together the EDL Timeline available here on Spaceflight101. EI occurred at 5:10:46 UTC (again, these are all actual Spacecraft UTC and NOT Earth Receive Time). Our timeline shows the same, so the reference point was spot on to get started. After EI, MSL went through the Guided Entry Process with events being purely velocity or G-Force triggered going through Pre-Bank, Range Control and Heading-Alignment. An interesting note that came out of today's press briefing was that the third bank reversal came very late into the Range Control Phase and there was not enough time before the start of the Heading Alignment explaining why Curiosity was not able to make a landing right in the middle of the ellipse. [This will be dealt with in an article further down the road when more data is available.]

Parachute Deployment came at 5:15:05 UTC (EI+4:19), our timeline shows 5:14:51 UTC, giving us a 14-second error, but it is worth noting that NASA JPL calculated a 10 to 20-second error for the Parachute Opening Time due to the dynamics of Entry [to add: JPL gave that error before atmospheric conditions and final tracking was available; their latest data bit which was not published had to be much closer since their final EDL-Simulation before the real event was done with much more accuracy]. With Parachute Deployment occurring right in its timeline window, it is safe to assume that it happened in the correct velocity regime of Mach 1.8 to 2.2. Heat Shield Separation occurred at 5:15:25 UTC, which is EI+4:39 and PD+0:20. In our pre-EDL predicts, it is shown at 5:15:20 UTC which is very close to the actual value, but the time between PD and HSS was estimated at 29 seconds giving us a larger error. An explanation for this is that PD might have occurred at a lower speed (e.g. closer to Mach 1.8), so that the time to reaching the required Mach number for HSS was shortened. Getting the detailed EDL data will give further insight into that. The next major event was Backshell Separation which was given as HSS+95 seconds in today's briefing, placing it at 5:17:00 UTC, EI+6:14 and PD+115 seconds. For the UTC, our pre-EDL estimate shows a 29-second error. For the Entry, Descent and Landing Sequence, the time under parachute was the biggest variable.

Credit: Ravi Prakash et al., MSL EDL System Overview,
IEEAC #1531

It could have been as short as 55 seconds and as long as 170 seconds, depending on the speed and altitude at parachute deployment and, of course, atmospheric conditions at the day of landing. In our timeline, the Time Under Parachute was estimated at 100 seconds [which was below the average Monte Carlo results] and during actual EDL, MSL spent 115 seconds under the parachute and was within the known margin. Taking these 15 seconds and adding the 14-second error of our HSS Time, we get those 29 seconds. During Monday's descent, MSL needed 57 seconds from Backshell Separation to Touchdown [if we take the confirmed touchdown time of 5:17:57, during Friday's Briefing a time of 55 seconds from BSS to TD was mentioned - this minor item will be sorted once all the data is back on the ground and analyzed]; our Pre-EDL prediction shows 54 seconds which is very good again, these 3 seconds are coming from a Constant Velocity Accordion that was basically non-existent (Preliminary calculation 0.15 seconds) and a longer Ready-for-Touchdown Phase. The altitude error at Constant Velocity Accordion was 3 meters, the Spaceflight101 Timeline had 2.5 seconds of CVA built-into it since the CVA margin was 0 to 5 seconds. The Ready to touchdown phase was estimated at >2 seconds in our timeline. At this point, the reconstruction becomes a bit shaky, and to avoid speculation, we'll stop here and look at it one more time when final results are provided by NASA.For the complete EDL Duration, there was a fairly large margin. The sequence could have been 380 to 460 seconds in duration. The Spaceflight101 prediction was 399 seconds (+/-40 seconds), once again, our scenario was a rather short EDL duration based on quick entry and short parachute flight. The actual EDL duration was 431 seconds [sticking to 5:17:57 for landing] which was longer than the average duration estimate acquired via early Monte Carlo Simulations, but still perfectly within margins. [Final Simulations done shortly before EDL by NASA/JPL were much closer to the actual duration, but these have not been available to the public.]

With these numbers, a reconstructed EDL Timeline can be produced.

*Preliminary* MSL EDL Timeline Reconstruction

Times given in red font represent confirmed data points based on NASA numbers. Times given in black font are times that represent time-triggered events (or such that can be given with +/-1 second accuracy) which can be deduced from NASA numbers. The grey times are remodeled times by Spaceflight101 taking different aspects into account that include the pre-EDL timeline, the NASA numbers and MARDI Imagery. Blue numbers are those from the pre-EDL timeline that remain unchanged for now until more data is available. The Sky-Crane portion of this timeline is still fairly uncertain and times will be adjusted when more data points are available.

***The complete timeline is subject to change as updated information becomes available and will be marked 'preliminary' until the entire EDL Sequence can be reconstructed with the complete data set that will be downlinked to Earth over the next few weeks.***

EI +/-

Event

UTC

ERT

-02:00:00

Final EDL Navigation Update

03:10

03:23

-13:30

HRS Propellant Vent

04:57:17

05:11:05

-10:00

Cruise Stage Separation

05:00:47

05:14:35

-09:30

Transition to EDL Communications:

05:01:17

05:15:05

-09:00

Descent Stage Reaction Control System Enable

05:01:47

05:15:35

De-Spin from 2rpm - Duration: 3min
6sec

-05:49

Entry
Guidance Enable, Center of Gravity Offset

05:05:17

05:19:05

-05:19

Quiescent Period, IMU Calibration

05:05:47

05:19:35

00:00

ENTRY INTERFACE

05:10:45.7

05:24:33.8

+00:39

Guided Entry Phases:

05:11:25

05:15:13

Pre-Bank (Initiation at 0.1G)

Range Control Phase (Initiation at 0.5G)

Heading Alignment (Initiation at 900m/s)

+01:25

Peak Heating - 2,100°C

05:12:11

05:25:59

+01:36

Peak Deceleration>11G

05:12:22

05:26:10

+04:04

Straighten Up and Fly Right (CG Offset Elimination)

05:14:50

05:28:38

+04:19

Parachute Deployment

05:15:04.9

05:28:53.0

+04:39

Heatshield Separation

05:15:24.6

05:29:12.7

+04:47

HSS+0:08

Begin Using
Landing Radar Solutions

05:15:33

05:29:21

+~05:00

Earth
Occultation: Tones unavailable

~05:15:46

~05:29:34

+06:04

BSS-0:10

Prime Mars Landing
Engines

05:16:50

05:30:38

+06:14

Backshell Separation

05:17:00

05:30:48

+06:14.8

BSS+0:00.8

Mars Landing Engines at 20%

05:17:01

05:30:49

+06:15

BSS+0:01

Mars Landing Engines at Full Throttle

05:17:01

05:30:49

+06:15.2

BSS+0:01.2

Attitude Control Only

05:17:01

05:30:49

+06:17.4

BSS+0:03.4

Powered Approach

05:17:03

05:30:51

+06:49

BSS+0:35

Constant Velocity Accordion - Duration: ~0.15sec

05:17:35

05:31:23

+06:48

Constant Deceleration to 0.75m/s

05:17:34

05:31:22

+06:50.5

Throttle down to 4 Mars Landing Engines

05:17:36

05:31:24

+06:53

Rover Separation, Deploy Front Rocker

05:17:38.6

05:31:26.7

+06:56

RS+0:03

Mobility Deploy

05:17:42

05:31:30

+06:57.5

RS+0:05.5

Release Bogie

05:17:44

05:31:32

+07:00

RS+0:07

Snatch

05:17:46

05:31:34

+07:02

RS+0:09

Enable Touchdown Logic, Differential Release

05:17:48

05:31:36

+07:11

Touchdown

05:17:57

05:31:45

Total EDL Duration (From EI to TD): 431 seconds

+07:11

TD+0:00:00

Touchdown - August 6, 2012 - 5:31 GMT (ERT)

05:17:57

05:31:45

+07:11.7

TD+0:01.7

All Wheels Down/Bridle Offload

05:17:59

05:31:47

+07:13.7

TD+0:02.7

Touchdown Detection

05:18:00

05:31:48

+07:14

TD+0:02.96

Bridle Cut

05:18:00

05:31:48

+07:14.2

TD+0:03.17

Umbilical Cord Cut Command

05:18:00

05:31:48

+07:14.2

TD+0:03.19

Umbilical Cord Cut

05:18:00

05:31:48

Fly Away Initiation - Flyaway: ~650m

+07:18.2

TD+0:07:19

Earliest Cutoff Time

05:18:04

05:31:52

This fantastic video, created by Brian Lynch, shows MSL's final descent to the Martian Surface. The video was created by combining the Mars Descent Imager thumbnail footage, a timeline of EDL Events and the audio from the Control Room at the Jet Propulsion Laboratory. Brian used our reconstructed EDL Timeline to supplement his own analysis of the MARDI images. Not all times of the video match those of our EDL Timeline because of known (minor) discrepancies between the time-stamps of the MARDI Images and the times given by NASA. Brian's video is based on the MARDI footage and points of reference deduced from the images while our EDL Reconstruction sticks to the official times provided by NASA.

1. Confusion about MSL's exact Landing Time (Added August 8, 2012)

After Curiosity made its historic landing on Mars, the focus of attention was obviously the Rover and its health status as well as the landing site and the first images being sent back to Earth. One aspect of the Landing that has shifted to the background is the exact timing of the safe touchdown made by Curiosity. Detailed Entry, Descent and Landing analysis are also yet to be completed, but the full engineering data set of EDL data has to be downlinked to the ground by the Rover first. Data downlink has already started and teams will perform initial performance reviews before starting to look at MEDLI Data to get a full picture of the EDL Sequence and the environment the vehicle encountered. Before the official and definitive touchdown time of 5:17:57 UTC on Monday, August 6, 2012 was confirmed, there was some confusion regarding the timing of landing. Immediately after landing, Curiosity sent an information package to the Odyssey Orbiter that was relaying EDL Communications to Earth. This short note of touchdown data included the time the Rover sensed touchdown at, the velocity that was detected at touchdown and the landing position using navigated trajectory data. This data was read out inside Mission Control as part of the Post-Landing Checklist. The landing time was given as 5:14:39 UTC (all times in this article are given in Spacecraft UTC and NOT Earth Receive Time) - which is roughly three minutes ahead of the timeline and is essentially impossible when looking at the EDL Design which allowed a margin of 80 seconds with EDL lasting for 380 to 460 seconds (from Entry Interface to Touchdown). [For those who'd like to convince themselves: This link sends you to the Ustream Recording of the EDL Coverage, 1:20:25 in you can hear the EDL Dynamics Report: "We have a touchdown time of ... uh.. 10:14:39 which is in PDT."]The official times calculated by NASA JPL several days before landing were 5:10:46 UTC for Entry Interface and 5:17:49 (+/-1 Minute) UTC for landing. Applying the 380 to 460 EDL duration to the EI time leads to a theoretical touchdown time at some point between 5:17:06 and 5:18:26 UTC. The EI-Time is very accurate and tracking during Landing Day revealed that Entry was occurring essentially spot-on without any major uncertainty being added to the EI-Time mentioned above.

Photo: NASA

Mission Control - Before and After

Photo: NASA

This puts the 5:14:39 landing off the table. What could have caused the wrong time being called out over the voice loops remains unclear. Whether Curiosity sent a false EVR (Event Record) or the time was simply called out wrong in all the excitement of EDL is not clear. Although, the latter or some type of mistake made on the ground (such as data processing or a conversion on-the-fly while sitting in mission control) seems to be more plausible. As a matter of fact, the timing of landing was reported wrong during the Post-Landing Press Conference when an overwhelmed Adam Steltzner said landing happened at 10:39 local time (Which was Earth Receive Time, but still wrong.). But who could blame him after having a long and nerve-wrecking day inside Mission Control.

Taking a look at the actual landing time of 5:17:57 UTC shows that MSL performed pretty much according to the two pre-EDL estimates which were 5:17:49 (+8 seconds) for NASA JPL and 5:17:25 for the Spaceflight101 Timeline (+32 seconds) both well within that margin of one minute. The Spaceflight101 estimate had a shorter 'Time Under Parachute' built-into it which was shorter than the average time of 112.5 seconds calculated for for time under parachute (pre-EDL). As EDL analysis continue, we will provide a reconstructed EDL Timeline as soon as sufficient data points are available.

2. Pin-Pointing the Landing Site (Added August 8, 2012)

Pin-Pointing the Landing site is relatively easy because NASA has a great asset in Orbit around Mars. After taking a spectacular Image during Entry, Descent & landing, the Mars Reconnaissance Orbiter followed up one day after landing with a Landing Site Image showing all major components of the Entry Vehicle.

Photo: NASA/JPL/University of Arizona

Distances to the Curiosity Rover:Heat Shield: 1,200 metersBack Shell & Parachute: 615 metersDescent Stage: 650 meters (In this image the descent stage is marked as 'Sky Crane', although, the flight hardware is called Descent Stage while the Sky Crane is the name of the landing technique.)

Pin-Pointing The Rover's Landing Site

image: NASA/JPL/Caltech

Credit: NASA/JPL

Credit: NASA/JPL-Caltech/MSSS/University of Arizona

Localizing the six Entry Ballast Masses

12 Kilometers from Rover, 1-Kilometer Dispersal

Credit: NASA/JPL-Caltech/MSSS

3. MARDI Thumbnail Analysis (Added August 9, 2012)

The Mars Descent Imager is a fixed focus full color camera fixed-body-mounted to the forward-port-side of the Rover. Its optical axis points in the +Z direction toward the ground. During Entry, Descent and Landing, the camera acquired high resolution imagery at a little more than four frames per second looking down to the ground. The full resolution images are 1600 by 1200 pixels. To learn more about the MARDI Hardware, visit its section in our MSL Landing Special.Currently, MSL has only downlinked thumbnail images of the full MARDI Frames. One full-resolution image was downlinked however. From the thumbnails, NASA JPL produced this video.All the thumbnails sent down to Earth include a time-stamp, allowing us to take a closer look at MSL's EDL Sequence. Again, these are only thumbnails and they allow only a limited number of events to be identified when looking at the set of images without starting excessive speculation.

Photo: NASA/JPL/MSSS

MARDI Image acquisition started at 5:15:21 UTC (Spacecraft time and NOT Earth Receive Time) with a black frame since there was no light passing inside the Aeroshell with the Heat Shield being still in place. The final image is time-stamped 5:21:48 UTC.

Here are the events that can be clearly identified by using the small thumbnail images:First sign of Heat Shield Separation comes in the first frame taken at 5:15:28 UTC. This time can be identified as Heat Shield separation time +/-1 second, putting it at Entry Interface +4 minutes and 42 seconds and Touchdown -2 minutes and 29 seconds. Checking back with our EDL Timeline, we can see a deviation of 8 seconds to the reference point of Entry Interface - there was a 10-20-second uncertainty for the parachute opening time and since HSS was velocity triggered, it was directly depending on the parachute deployment time. (The Time to Touchdown shows a bigger error, but that was due to the 'Time under Parachute' being longer than anticipated.) The first Frame at 5:15:28 shows the MARDI Calibration Target close-up with poor lighting since the Heat Shield just separated shortly before that - not allowing much light to reach the inside. The second frame is a good image of the target while the third frame shows the edge of the Heat Shield, the target and wiring associated with the MEDLI suite of instruments. The first frame showing the complete heat shield is frame 2 of 5:15:29. In subsequent frames, the Heat Shield gets smaller relatively quickly, showing that the separation at positive relative velocity of the Heat Shield was accomplished very well.

5:15:28 UTC - All Frames

Photo: NASA/JPL/MSSS

Frame 1

Photo: NASA/JPL/MSSS

Frame 2

Photo: NASA/JPL/MSSS

Frame 3

Photo: NASA/JPL/MSSS

Frame 4

The only full MARDI Frame (2nd frame of 5:15:30) that has been downlinked to date (August 9, 2012) shows the heat shield as it drops away being at a distance of 15 meters. It also shows a darker area of martian Soil below the vehicle. So, the heat shield was already at a distance of 15 meters two seconds after separation.This area is a great reference to see the motion the vehicle goes through in subsequent frames. Over a period of ~70 seconds, this region of darker material does a "turn" of ~~270 degrees. At some point during this time, the backshell separated and powered flight would have started and there are some frames in the series that look suspicious, but that would be speculation.

Photo: NASA/JPL/MSSS

The next event that can be clearly seen in the small thumbnails is dust being picked up by the Mars Landing Engines. That is first visible in the second frame of 5:17:35 UTC. The first frame looks looks fairly clean, but it is definitely possible that the full frames will reveal dust being picked up somewhat earlier.

Photo: NASA/JPL/MSSS

5:17:35 UTC

Photo: NASA/JPL/MSSS

5:17:41 UTC

The final major event is the deployment of the Mobility System. Because of its position, MARDI was able to observe Mobility Deploy by seeing a wheel pop into view. This occurs in the third frame taken at 5:17:43 UTC - roughly 14 seconds before landing which is right on the timeline being estimated at an altitude of 18.6 meters ahead of EDL. Actually seeing the wheel move outward at Touchdown is fairly hard when looking at the raw thumbnail images. According to NASA, the official and exact landing time was 5:17:57.3 UTC. Post Touchdown MARDI Images show a single brighter line of Martian Soil while the rest of the frame is dark since the Rover's structure was blocking sunlight. To Recap: The MARDI Thumbnails have given us the following data points:1) Heat Shield Separation: 5:15:28 +/-1 second - Pretty much on the nominal EDL Timeline.2) Mobility Deploy: 5:17:43 +/-2 seconds - Taking the time of touchdown as reference, it is perfectly in sequence with the pre-EDL outline.

Mobility Deploy

Photo: NASA/JPL/MSSS

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